chemical warfare, employment in war of incendiaries, poison gases, and other chemical substances. Ancient armies attacking or defending fortified cities threw burning oil and fireballs. A primitive type of flamethrower was employed as early as the 5th cent. BC; modern types are still in use. In the Middle Ages, before the introduction of gunpowder, a flammable composition known as Greek fire was used. Smoke from burning straw or other material was employed in early times, but its effectiveness is uncertain.
Poison gas was first used during World War I, when the Germans released (Apr., 1915) chlorine gas against the Allies. The Germans also introduced mustard gas later in the war. Afterward, the major powers continued to stockpile gases for possible future use and several actually used it: the British in Afghanistan, the French and Spanish in Africa, the Italians in Ethiopia, and the Japanese in China. Lethal gases were not employed in combat during World War II, but the Germans did use gases for mass murder during the Holocaust. The Germans also invented and stockpiled the first nerve gas. It is odorless and colorless and attacks the body muscles, including the involuntary muscles. It is the most lethal and insidious weapon of chemical warfare. Since World War II, chemical weapons are known to have been used by Egypt in Yemen (during the 1962–67 civil war) and by Iraq against Iran during the Iran-Iraq War and against Kurdish rebels. Iraq threatened to use them in the First Persian Gulf War, but it is unclear whether their use was attempted or not. Syria was accused of using chemical weapons in the civil war that began in 2011; it denied the charges and accused rebels of using chemical weapons. An especially deadly attack in Aug., 2013, was linked by Western governments to the Syrian government; the subsequent threat of U.S. air strikes led Syria to agree to the destruction of its chemical weapons stockpile. Syria has also used chlorine gas as a weapon; the gas was not on the list of chemical weapons it had to declare.
Besides lethal gases, which attack the skin, blood, nervous, or respiratory system and require hospitalization of the victim, there are also nonlethal incapacitating agents, which, like tear gas, cause temporary physical disability. Such agents have often been employed in riot control, espionage, and warfare. Various forms of herbicides and defoliants are also used to destroy crops or vegetation, as Agent Orange was used by the United States during the Vietnam War.
The potential effectiveness of chemical warfare is increasing with improved methods of dissemination, such as artillery shells, grenades, missiles, and aircraft and submarine spray guns. Some protection against chemical weapons is possible using suits, sealed vehicles, and shelters. Such countermeasures usually protect against nuclear fallout and biological warfare as well. Lethal chemical weapons are held by many nations and they continue to be used. The danger of the proliferation of chemical and biological weapons remains despite arms control because they are relatively easy to manufacture and deploy.
Efforts to control chemical and biological weapons began in the late 19th cent. The Geneva Protocol of 1925, which went into force in 1928, condemned the use of chemical weapons but did not ban the development and stockpiling of chemical weapons. The United States did not ratify the protocol until 1974. In 1990, with the end of the cold war, the United States and the Soviet Union agreed to cut their arsenals by 80% in an effort to create a climate of change that would discourage smaller nations from stockpiling and using such lethal weapons. In 1993 a international treaty banning the production, stockpiling (both by 2007), and use of chemical weapons and calling for the establishment of an independent organization to verify compliance was adopted. The agreement, which became effective in 1997, has been signed and ratified by all but a handful of nations. The treaty is enforced by the Organization for the Prohibition of Chemical Weapons. The alleged Iraqi retention, after the Persian Gulf War cease-fire, of chemical weapons and other weapons of mass destruction was the main pretext for the 2003 U.S.-British invasion of Iraq.
See the ongoing Stockholm International Peace Research Institute (SIPRI), The Problems of Chemical and Biological Warfare (1971–); R. Harris and J. Paxman, A Higher Form of Killing (1982); E. M. Spiers, Chemical Warfare (1986); J. B. Tucker, War of Nerves (2006).